Azobenzene-siloxane hybrids with lamellar structures from bridge-type alkoxysilyl precursors

Article abstract of DOI:10.1039/c4ra01709a

Lamellar azobenzene-siloxane hybrids were prepared by controlled hydrolysis and polycondensation of three types of precursors, where azobenzene is sandwiched by mono-, di- and triethoxysilyl groups using propylene linkers. All precursors underwent reversible and fast trans-cis isomerization upon UV/Vis irradiation in dilute solution. Upon hydrolysis of the triethoxysilylated precursor in a homogeneous solution under acidic conditions, precipitation occurred by self-assembly of hydrolyzed monomers into a lamellar structure. Although di- and mono-ethoxysilylated precursors produced less ordered products under identical conditions, highly ordered lamellar films were obtainable either by evaporation induced self-assembly of the hydrolyzed monomers or by solid-state reactions of precursor films. The degree of trans-cis isomerization of azobenzene moieties in the hybrid films was enhanced by decreasing the cross-linking degree of siloxane networks using precursors with less condensable alkoxy groups.

Full text of DOI:10.1039/c4ra01709a

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PAPER
Azobenzene–siloxane hybrids with lamellar
structures from bridge-type alkoxysilyl precursors†
Cite this: RSC Adv., 2014, 4, 25319
a
a
a
b
Sufang Guo, Watcharop Chaikittisilp, Tatsuya Okubo and Atsushi Shimojima*
Lamellar azobenzene–siloxane hybrids were prepared by controlled hydrolysis and polycondensation of
three types of precursors, where azobenzene is sandwiched by mono-, di- and triethoxysilyl groups
using propylene linkers. All precursors underwent reversible and fast trans–cis isomerization upon UV/Vis
irradiation in dilute solution. Upon hydrolysis of the triethoxysilylated precursor in a homogeneous
solution under acidic conditions, precipitation occurred by self-assembly of hydrolyzed monomers into a
lamellar structure. Although di- and mono-ethoxysilylated precursors produced less ordered products
under identical conditions, highly ordered lamellar films were obtainable either by evaporation induced
self-assembly of the hydrolyzed monomers or by solid-state reactions of precursor films. The degree of
trans–cis isomerization of azobenzene moieties in the hybrid films was enhanced by decreasing the
cross-linking degree of siloxane networks using precursors with less condensable alkoxy groups.
Received 26th February 2014
Accepted 27th May 2014
DOI: 10.1039/c4ra01709a
www.rsc.org/advances
precursors have the ability to incorporate a wider variety of
organic groups in the siloxane networks, providing greater
1
Introduction
Constructing inorganic–organic hybrid nanoarchitectures opportunities to create novel functional hybrid materials.
requires the integration of inorganic and organic building
Azobenzene is a typical photo-responsive molecule that
1,2
blocks on molecular or nanometer-length scales. Organo- undergoes photochemical conversion between trans and cis
siloxane is an excellent example of such hybrid materials, pos- isomers, accompanied by large changes in molecular shape and
sessing the benets of siloxane networks, such as good thermal/ size. The arrangement and mobility of azobenzene critically
chemical stabilities and high transparency, along with diverse inuence the photo-responsive properties of azobenzene-con-
functionalities endowed by organic moieties. The preparation taining materials. Azobenzene–siloxane hybrids, produced by
of organosiloxane-based materials with unique properties, self-directed assembly of azobenzene-containing alkoxysily
2–4
structures and morphologies has been widely studied.
precursors, are a potential new class of photo-responsive mate-
Sol–gel hydrolysis and polycondensation of organo- rials; however, their synthesis and properties have not yet been
alkoxysilanes provides a convenient route to obtaining orga- fully investigated. Liu et al. reported the synthesis of a photoactive
nosiloxane hybrids; however, this process generally yields azobenzene-bridged alkoxysilane containing urea (–NH–CO–NH–)
13
hybrids with amorphous structures. In recent years, careful linkages. This precursor can self-assemble into lamellar struc-
precursor designing and optimization of the reaction condi- tures through H-bonding interaction between urea groups and p–
tions have produced organosiloxane hybrids with ordered p interaction between azobenzene groups. However, trans–cis
structures, such as lamellar and two-dimensional hexagonal isomerization is almost inhibited in the solids possibly due to
structures. Organoalkoxysilanes with pendant and bridging intense H-bonds. Azobenzene-bridged alkoxysilanes containing
0
0
0
, respec- simple alkylene linkers are worth investigating as a means of
3
organic (R) groups (R–Si(OR )
tively) are commonly used. The components of organic moieties improving their photo-responsive properties. Such precursors
R) are extremely important for self-assembly and therefore to were used to produce photo-responsive mesoporous silica, where
direct the formation of ordered structures. Non-covalent inter- azobenzenes were randomly arranged in the pore walls, by
3
and (R O)
3
Si–R–Si(OR )
(
14,15
actions, such as hydrophobic interactions, strong H-bonds (e.g. surfactant-directed self-assembly with additional silica source.
urea and amide linkages) and p–p stacking interactions, oen As yet, their self-directed assembly, as well as photo-responsive
favour self-directed assembly.
5–12
In general, bridge-type properties of the products, are unexplored.
In this study, ordered azobenzene–siloxane hybrid materials
a
were prepared by self-directed self-assembly of azobenzene-
bridged alkoxysilanes containing propylene linkers. Three types
Department of Applied Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, of precursors with tri-, di- and mono-ethoxysilyl groups (1, 2 and
Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
b
Tokyo 169-8555, Japan. E-mail: shimojima@waseda.jp
3 in Scheme 1) were used to study the effects of (i) the number of
Si–OH groups on self-assembly and (ii) the degree of Si–O–Si
†
Electronic supplementary information (ESI) available: Fig. S1–S8 and Scheme
S1. See DOI: 10.1039/c4ra01709a
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0
Synthesis of 4,4 -diallyloxyazobenzene
0
In a 100 mL Schlenk ask, 4,4 -dihydroxyazobenzene (1.00 g,
4
(
.66 mmol) was dissolved in 10 mL of DMF. Activated NaH
0.56 g, 14.0 mmol), dispersed in DMF (20 mL), was added to the
ask. Aer stirring at room temperature for 2 h, allyl bromide

(
6
3.49 g, 11.7 mmol) was added, and the mixture was stirred at
0 C for 1 day under nitrogen atmosphere. The product was
ꢁ
extracted with ethyl acetate, washed with cold water and dried
over MgSO . Evaporation of ethyl acetate gave a dark red, crude
4
solid product. Yellow crystals (0.64 g; yield of 47%) were
obtained aer recrystallization from ethyl acetate. H NMR (d,
Scheme 1 Structures of bridged-type 1, 2 and 3 precursors and the
preparation pathways of hybrid powders and films.
1
270 MHz, CDCl
3
): 4.60–4.61 (d, 4H, ArOCH
2
), 5.30–5.48 (m, 4H,
CH ]CH), 6.10–6.15 (m, 2H, ArCH
2
2
]CH), 6.98–7.04 (d, 4H,
1
3
cross-linking on photo-isomerization of bridged azobenzene ArH), 7.84–7.89 (d, 4H, ArH). C NMR (d, 67.8 MHz, CDCl
groups. Powder samples (P1, P2 and P3) were obtained as 69.04, 114.94, 118.00, 124.33, 132.87, 147.18, 160.60.
precipitates by acid-catalyzed hydrolysis and polycondensation
3
):
in THF–H O solutions. Furthermore, to ensure high-efficiency
0
2
Synthesis of 4,4 -[3-(triethoxysilyl)propoxy]azobenzene (1)
13,16–21
light absorption,
thin lms were prepared on glass
0
Hydrosilylation of 4,4 -diallyloxy-azobenzene (0.294 g, 1.00 mmol)
in an excess amount of triethoxysilane (3.28 g, 20.0 mmol) was
performed in toluene (20 mL) in the presence of a Pt catalyst
substrates by either an evaporation induced self-assembly
0
0
(
EISA) technique (F2 and F3) or solid-state reactions (F1 , F2
0
and F3 ). The trans–cis isomerization of azobenzene groups in
hybrid thin lms upon UV/Vis irradiation was investigated.
ꢃ5
ꢁ
(
0.027 g, 2 ꢂ 10 mol). The mixture was stirred at 70 C for 24 h
under nitrogen atmosphere, and the solvent and unreacted
triethoxysilane were removed in vacuo. 1 was obtained as red
crystals (0.529 g, yield of 85%) aer purication using gel
2
Experimental
permeation chromatography (GPC), with chloroform as the
eluent. H NMR (d, 270 MHz, CDCl ): 0.76–0.82 (m, 2H,
OCH CH CH Si), 1.21–1.26 (m, 9H, Si(OCH CH ) ), 1.78–2.00 (m,
CH
CH
H, ArH). C NMR (d, 67.8 MHz, CDCl
Materials
1
3
Allyl bromide (>98.0%), 4-aminophenol (>98.0%), phenol
2
2
2
2
3 3
(
>99.0%), sodium nitrite (>98.5%), hydrochloric acid (HCl, 1 M),
sodium hydroxide (NaOH, >97.0%), N,N-dimethylformamide
DMF, dehydrated, >99.5%), ethyl acetate (>99.0%) and dime-
2H, OCH CH
2 2
2 2 3 3
Si), 3.81–3.89 (m, 6H, Si(OCH CH ) ), 4.00–4.05
(
2
7
m, 2H, OCH
2
2
CH
2
Si), 6.96–7.00 (d, 2H, ArH), 7.83–7.88 (d,
): 6.50, 18.32, 22.77, 58.46,
(
13
3
thylethoxysilane were purchased from Wako Pure Chemical
Industry. Sodium hydride (NaH) (60% dispersion in paraffin
liquid), triethoxysilane (>97.0%) and diethoxymethysilane
29
0.13, 114.67, 124.30, 146.95, 161.11. Si NMR (d, 53.45 MHz,
+
CDCl
3
): ꢃ45.53. ESI-MS: m/z: 645.2994 [M + Na] .
(
>95%) were purchased from Tokyo Chemical Industry. Plat-
0
Synthesis of 4,4 -[3-(diethoxymethylsilyl)propoxy]azobenzene (2)
inum (0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complexes in
0
xylene (Pt ꢀ2%), sulfamic acid ($99.5%) and acetic acid The chemical 4,4 -diallyloxyazobenzene (0.294 g, 1.00 mmol)
(
$99.5%) were purchased from Sigma-Aldrich. All chemicals dissolved in toluene (15 mL) was mixed with diethoxy-
ꢃ5
were used as received, without further purication.
methylsilane (2.68 g, 20.0 mmol) and 0.02 g (2 ꢂ 10 mol) of a
Pt catalyst. The mixture was stirred under nitrogen atmosphere
ꢁ
0
at 70 C for 24 h. Yellow crystals (0.51 g, yield of 90%) were
obtained aer solvent evaporation, followed by purication
using GPC. H NMR (d, 270 MHz, CDCl
.74–0.80 (m, 2H, OCH CH CH Si), 1.20–1.28 (m, 6H,
Si(OCH CH ) ), 1.75–1.95 (m, 2H, OCH CH CH Si), 3.75–3.83
m, 4H, Si(OCH CH ) ), 3.98–4.03 (m, 2H, OCH CH CH Si),
.95–7.01 (d, 2H, ArH), 7.83–7.89 (d, 2H, ArH). C NMR (d, 67.8
MHz, CDCl ): ꢃ4.87, 9.96, 18.42, 22.82, 58.19, 70.36, 114.67,
24.30, 146.96, 161.08. Si NMR (d, 53.45 MHz, CDCl
ESI-MS: m/z: 585.2786 [M + Na] .
Synthesis of 4,4 -dihydroxyazobenzene
22
The procedure was conducted as per a previous report, with
slight modication. 4-Aminophenol (2 g, 18.3 mmol) and 16%
HCl (8.4 mL) were added to a 20 mL ask (ask A). Aer being
stirred for 45 min at room temperature, the mixture was cooled
to 0 C. Then, a 2 M NaNO solution was slowly added while
1
3
): 0.163 (s, 3H, SiCH
3
),
0
2 2 2
2
3 2
2
2
2
(
6
2 3 2 2 2 2
ꢁ
2
13
ꢁ
ꢁ
stirring at 0 C. The mixture was stirred at 0 C for another 2 h,
followed by the addition of sulfamic acid to remove the excess
3
2
9
1
3
): ꢃ5.08.
NaNO
was mixed with a 2 M NaOH solution (1.47 g, 36.6 mmol in
7 mL H O). The solution in ask A was then added to ask B,
and subsequently stirred at room temperature for 10 h. The
crude product was recrystallized in an EtOH–H
2
. In a 50 mL ask (ask B), phenol (1.72 g, 18.3 mmol)
+
1
2
0
Synthesis of 4,4 -[3-(ethoxydimethylsilyl)propoxy]azobenzene (3)
0
O solution (1 : 5 4,4 -Diallyloxy-azobenzene (0.294 g, 1.00 mmol) dissolved in
2
1
v/v), yielding a 1.56 g dark red product (40% yield). H NMR (d, toluene (15 mL) was mixed with diethoxymethylsilane (2.08 g,
2
ꢃ5
70 MHz, DMSO-d ): 6.89–6.93 (d, 4H, ArH), 7.70–7.80 (d, 4H, 20.0 mmol) and 0.02 g (2 ꢂ 10 mol) of a Pt catalyst. The
6
1
3
ꢁ
ArH), 10.13 (s, 2H, Ar–OH). C NMR (d, 67.8 MHz, DMSO-d
15.76, 124.12, 145.25, 159.96.
6
): mixture was stirred under nitrogen atmosphere at 70 C for
1
24 h. Yellow crystals (0.43 g, yield of 85%) were obtained aer
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solvent evaporation, followed by purication using GPC.
H
observed on a eld-emission scanning electron microscope (FE-
NMR (d, 270 MHz, CDCl ): 0.151 (s, 6H, Si(CH ) ), 0.71–0.77 (m, SEM, Hitachi S-900) with an accelerating voltage of 6 kV. Before
3
3 2
2
H, OCH CH CH Si), 1.18–1.23 (m, 3H, SiOCH CH ), 1.82–1.93 observation, samples were sputter-coated with Pt. A SUPER-
2 2 2 2 3
(
4
m, 2H, OCH CH CH Si), 3.65–3.68 (m, 2H, SiOCH CH ), 3.99– CURE-204S UV light source (San-ei electric) with an intensity of
2 2 2 2 3
ꢃ2
.04 (m, 2H, OCH
2
CH
2
CH
2
Si), 6.97–7.01 (d, 2H, ArH), 7.84–7.88 67–76 mW cm was employed for UV and visible light irradi-
): ꢃ2.10, 12.39, 18.57, ation of the samples. UV cut (HOYA L-420 nm) and UV pass
1
3
(d, 2H, ArH). C NMR (d, 67.8 MHz, CDCl
3
2
9
2
5
3.15, 58.34, 70.64, 114.67, 124.31, 146.96, 161.10. Si NMR (d, (HOYA U-340 nm) lters were used. Thermogravimetry (TG)-
+
3.45 MHz, CDCl
3
): 17.18. ESI-MS: m/z: 525.2576 [M + Na] .
differential thermal analysis (DTA) measurements were per-
formed on Rigaku 8120 in helium atmosphere with a heating
ꢃ1
rate of 5 K min
.
Preparation of hybrid powders (P1, P2 and P3)
An HCl aqueous solution was added to the THF solutions of 1, 2
and 3, yielding molar compositions of 1 : THF : H
2
O : HCl ¼
3
Results and discussion
1
3
: 50 : 15 : 0.03, 2 : THF : H
2
O : HCl ¼ 1 : 50 : 10 : 0.02, and
: THF : H O : HCl
¼
1 : 50 : 10 : 0.02, respectively. The Characterizations of precursors 1, 2 and 3
2
mixtures were stirred at room temperature for several hours.
The bridged-type precursors 1, 2 and 3 have similar structures.
They possess the same ‘bridge’ (organic fragment between the
two Si atoms, having a length of ca. 1.9 nm) while differ in the
number of ethoxy groups attached to each Si atom (3, 2 and 1 for
1, 2 and 3, respectively).
Powder XRD patterns of the precursors 1, 2 and 3 are shown
in Fig. 1. The rst strong peaks are attributed to the diffractions
from crystals in one orientation, corresponding to the d-spac-
ings of 1.42, 1.70 and 1.93 nm, respectively. Differences in the d-
spacings indicate the varying arrangement and/or orientation of
the organic bridges. TG-DTA curves of 1, 2 and 3 show endo-
thermic peaks at ca. 85 C, 60 C and 79 C, respectively, without
weight losses (Fig. S1 in ESI†), which is attributed to the melting
of the molecular crystals.
These precursors can also be obtained as thin lms by spin-
coating their THF solutions on glass substrates. In these lms,
lamellar structures are highly orientated relative to the surface,
as suggested by the disappearance of diffraction peaks at high
angles (Fig. S2 in ESI†). Polarized optical microscopic images of
The resulting precipitates were collected by ltration and
ꢁ
subsequently heated overnight at 120 C, giving P1, P2 and P3.
Preparation of hybrid lms (F2 and F3) by EISA processes
An HCl aqueous solution was added to THF solutions of 2 and 3
(
3
molar ratios of 2 : THF : H
2
O : HCl ¼ 1 : 100 : 10 : 0.02 and
: THF : H
2
O : HCl ¼ 1 : 100 : 10 : 0.005). The mixtures were
stirred at room temperature for 2 h and 1 h, respectively. A
portion of the mixtures was spin-coated on glass substrates to
produce thin lms. To induce further polymerization, the lm
prepared from 2 was exposed to 5 M HCl vapour for 30 min, and
ꢁ
ꢁ
ꢁ
ꢁ
the lm prepared from 3 was heated at 120 C for 3 h to give F2
and F3, respectively. Note that lm samples cannot be obtained
from 1 by this procedure due to precipitation upon hydrolysis.
0
0
0
Preparation of hybrid lms (F1 , F2 and F3 ) by solid-state
reactions
First, crystalline thin lms of precursors 1, 2 and 3 were
prepared by spin-coating (3000 rpm) their THF solutions (ca.
1, 2 and 3 lms (Fig. S3 in ESI†) depict birefringent texture of
crystals, indicating submicroscopic ordered structures.
0 0 0
0
.25 M) on glass substrates. Subsequently, F1 , F2 and F3 lms
The trans–cis photo-isomerization properties of precursors
were obtained by solid–liquid reactions, that is, by treating the
precursor lms in a 0.1 M HCl solution at room temperature for
1–3 were investigated by UV-Vis spectra of their dilute EtOH
ꢃ5
solutions (ca. 5 ꢂ 10 M) upon UV/Vis irradiation. As depicted
in Fig. 2, all precursors before irradiation exhibited large
absorption peaks at ca. 360 nm, due to p–p* transitions of
0
0
0
1
5 days (F1 ) and 9 days (F2 and F3 ), followed by air drying.
Characterizations
1
13
29
Solution-state H-, C- and Si-NMR spectra were recorded on
a JEOL JNM-270 spectrometer at 270, 67.8 and 53.45 MHz,
respectively. DMSO-d or CDCl was used as the solvent and
6
3
tetramethylsilane as the internal reference. High resolution ESI
mass analysis was carried out with a JEOL JMS-T100 CS
instrument. Samples were dissolved in methanol. X-ray
diffraction (XRD) patterns were obtained using a Rigaku Ultima
IV diffractometer operated at 40 kV and 40 mA, with CuKa
radiation (0.15406 nm). The measurements were performed
ꢁ
ꢁ
ꢁ
ꢃ1
over a 2q range of 2–30 at scanning speeds of 2 and 1 min
ꢁ
with a step width of 0.02 for precursors and hybrid materials,
respectively. Fourier transform infrared (FT-IR) spectra were
recorded using a JASCO FT/IR-6100 spectrometer by the KBr
pellet technique. UV-Vis absorption spectra were recorded on a
JASCO V-670 instrument. Morphologies of the samples were Fig. 1 Powder XRD patterns of the precursors: (a) 1, (b) 2 and (c) 3.
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thermally stable trans isomers. Aer 1 min of UV irradiation, the
peaks signicantly decreased, while the broad absorption peaks
at ca. 440 nm due to n–p* transitions of cis isomers increased. A
further 1 min of Vis irradiation almost recovered the spectra to
the original states, in which trans isomers are dominant. In all
cases, 1 min of UV or Vis irradiation was enough to drive azo-
benzenes to reach a stable isomerization state, suggesting that
reversible, fast trans–cis photo-isomerization can be achieved in
dilute EtOH solutions of 1, 2 and 3 precursors.
Fig. 3 FE-SEM images of (a) P1, (b) P2 and (c) P3 (scale bar: 1 mm).
Hybrid materials prepared by hydrolysis and
polycondensation of precursors 1, 2 and 3
Hybrid powders (P1, P2 and P3). FE-SEM images of P1, P2
and P3 are shown in Fig. 3. P1 consists of plate-like particles,
whereas P2 and P3 exhibited irregular morphologies. Powder
XRD patterns of P1, P2 and P3 are shown in Fig. 4. An ordered
lamellar structure with the d-spacing of 2.20 nm was observed
for P1. Well-resolved diffraction peaks from the rst to third
order revealed the structure with a long-range order. The
increased d-spacing, compared to the precursor crystals (cf. Fig. 4 Powder XRD patterns of (a) P1, (b) P2 and (c) P3.
Fig. 1 and S2†), may be caused by a different arrangement of the
organic bridges. On the other hand, P2 and P3 showed no peak
and a small broad peak (d ¼ 1.66 nm), respectively, at low 2q
ꢃ1
ca. 3245 cm can be assigned to the O–H stretching modes of
regions, indicating less ordered structures with a short-range
SiOH and physisorbed water. No peak of isolated, free silanol
order if present. The additional peaks at higher 2q region might
ꢃ1 25
groups (ꢀ3750 cm
)
was observed. The as-formed precipi-
be attributed to the spacings between the organic bridges and/
tates have a highly ordered lamellar structure (Fig. S4(a), in
23,24
or to some ordering in the siloxane networks,
details are not understood.
although
ESI†), which is probably stabilized by H-bondings of SiOH
26
groups. Aer heating (Fig. 5 (a, top line)), the absorption
bands of SiOH decreased greatly, accompanied by an increase
in Si–O–Si absorption bands (e.g. an asymmetric stretching
To obtain information about the progress of hydrolysis and
polycondensation, FT-IR spectra of P1, P2 and P3 were
compared with those of the precursors and as-formed precipi-
tates (i.e. P1, P2 and P3 before heating). The as-formed precip-
ꢃ1
band of Si–O–Si at 1023 cm ). On the other hand, for P2 and
P3, both hydrolysis and polycondensation proceeded to a high
extent even prior to heating, as judged from the intense Si–O–Si
bands and relatively small SiOH bands (Fig. 5 (middle lines of
(b) and (c), respectively). No signicant changes were observed
aer heating (Fig. 5 (top lines of (b) and (c)).
itates derived from 1 (Fig. 5 (a, middle line)) exhibit no
ꢃ1
OCH
2
CH
3
absorption peaks (2995 cm , cf. Fig. 5 (a, bottom
line) for 1), suggesting that the ethoxysilyl (SiOEt) groups of 1
ꢃ1
were fully hydrolyzed. The new peak at 896 cm was ascribed to
the silanol (SiOH) groups. The large absorption band centred at
The ordered structure of P1 was considered to be formed by
self-assembly of amphiphilic hydrolyzed species through
hydrophobic (and/or p–p) interactions between organic bridges
and H-bondings between silanol groups. Three steps, i.e.
hydrolysis, self-assembly (precipitation) and polycondensation,
should proceed sequentially, as supported by the above results.
In the cases of less-ordered P2 and P3, polycondensation likely
occurred in the solutions, forming precipitates without self-
assembly. The difference is attributed to the number of SiOH
groups; precursors 1, 2 and 3 can form 6, 4 and 2 SiOH groups,
respectively, aer complete hydrolysis. The lack of self-assembly
ability of 2 and 3 may be due to the absence of adequate H-
bonding interactions to stabilize lamellar structures, leading to
the formation of less ordered structures.
Hybrid lms (F2 and F3) prepared by EISA. EISA is a
convenient method to prepare lms with highly ordered mes-
2
7,28
ostructures from silicate–surfactant mixtures
lyzed organoalkoxysilanes.
and hydro-
4,29
The preparation of a thin lm
Fig. 2 UV-Vis spectra of EtOH solutions of precursors (a) 1, (b) 2 and
from 1 by EISA was unsuccessful due to the precipitation of
hydrolyzed monomers. However, F2 and F3 lms with lamellar
(
c) 3: before irradiation (red line), after 1 min of UV irradiation (blue line),
and after a further 1 min of Vis irradiation (green line).
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Fig. 6 XRD patterns of (a) F2 and (b) F3.
reactions occur at the interface of two phases. In this process,
morphologies or the ordered structures of the precursors are
30,31
prone to be preserved.
The reaction may be hindered by a
slow diffusion of reactive agents into solid phases, especially
aer siloxane formation. In this work, solid–liquid reactions of
1, 2 and 3 precursor lms in HCl aqueous solutions (0.1 M) were
conducted. Reaction rates were relatively slow compared to
solution-phase reactions; it was conrmed that ethoxy groups
were almost eliminated aer 9 days in the cases of 2 and 3 but
0
0
additional 6 days were required for 1. FT-IR spectra of F1 –F3
(
(
Fig. S6 in ESI†) exhibited absorption peaks for Si–O–Si
ꢃ1
1060 cm ), indicating the progress of hydrolysis and poly-
ꢃ1
condensation, although peaks for the SiOH (900 cm ) groups
still remained to some extent.
0
0
The XRD patterns of F1 –F3 lms are shown in Fig. 7. All
lms showed lamellar structures, with d ¼ 2.28, 1.85 and

Fig. 5 FT-IR spectra of (a) precursor 1 (bottom), as-formed precipi-
tates (middle) and P1 (top), (b) precursor 2 (bottom), as-formed
0 0 0
1
.99 nm for F1 , F2 and F3 , respectively. Relatively weak peaks
0
0
precipitates (middle) and P2 (top) and (c) precursor 3 (bottom), as- in F1 and F2 indicated less ordered structures, likely due to a
formed precipitates (middle) and P3 (top).
partial loss of the crystalline arrangement of precursor molecules
0
0
during the solid-state reaction. The d-spacings of F2 and F3 were
similar to those of the precursor lms (1.89 nm and 1.93 nm,
Fig. S2 in ESI†) indicating the almost unchanged molecular
arrangements, despite the conversion of SiOEt to Si–O–Si.
structures were successfully prepared by spin-coating the clear
solutions containing hydrolyzed 2 and 3, followed by HCl
vapour treatment and heating, respectively, to promote poly-
condensation. It should be noted here that heating of as-coated
F2 caused disordering of its structure; hence, acid vapour
Photo-responsive properties of the hybrid lms
0
treatment was applied instead. FT-IR spectra of these lms prior The photo-responsive properties of the hybrid lms (F2, F3, F1 ,
0
0
to the treatment show strong SiOH peaks, suggesting as yet no F2 and F3 ) were investigated by measuring their UV-Vis spectra
obvious polycondensation (Fig. S5(b) in ESI†). This differs from upon light irradiation. We have recently reported that azo-
their corresponding precipitates, in which no obvious SiOH benzene–siloxane hybrid lms prepared from pendant-type
bands are evident (Fig. 5, middle lines of (b) and (c)). Aer acid precursors showed partial but reversible trans–cis isomeriza-
16
treatment or heating of these lms, the SiOH bands decreased, tions. Bridged-type hybrids may suffer from a low mobility of
accompanied by the appearance of Si–O–Si bands at ca. bridging azobenzene groups due to greater constraints at both
ꢃ1
1
060 cm (Fig. S5(c) in ESI†). Fig. 6 shows that F2 and F3 show
sharp diffraction peaks due to lamellar structures with the d-
spacings of 1.81 and 1.59 nm, respectively, in contrast to the
broad peaks observed for P2 and P3 precipitated from the
hydrolyzed solutions. It is apparent that fast evaporation of the
solvents facilitated the self-assembly of hydrolyzed species prior
to polycondensation. The additional small, sharp peaks (2q ¼
ꢁ
ꢁ
ca. 8 and 10.5 ) suggest the presence of other phases, which are
unidentied yet.
0
0
0
Hybrid lms (F1 , F2 and F3 ) prepared by solid-state reac-
tions. In solid-state reaction, reactive vapour or liquid diffuses
0
0
0
into solids and subsequent hydrolysis and polycondensation Fig. 7 XRD patterns of (a) F1 , (b) F2 and (c) F3 .
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ends, thereby limiting trans–cis isomerization. It is expected
that a decrease in the degree of Si–O–Si cross-linking by
utilizing precursors with less SiOEt groups will increase the

exibility of siloxane networks so that the mobility of azo-
benzene groups can be improved.
As azobenzenes are less mobile in lms than in solutions, a
longer time (5 min) of irradiation (1 min for solutions) was
conducted. As depicted in Fig. 8(a), F2 shows an apparent
increase in absorption at 450 nm upon UV irradiation, indi-
cating a partial of trans to cis isomerization. However, absorp- Fig. 9 XRD patterns of (a) F2 and (b) F3: (bottom) before irradiation,
(
middle) after 5 min of UV irradiation, (top) after a further 5 min of
tion at 350 nm also increased aer UV irradiation, which is
possibly due to an orientation change of azobenzene moieties
relative to incident light caused by order–disorder transition
upon irradiation, as described later. In contrast, F3 (Fig. 8(b))
showed obviously reversible trans–cis isomerization upon UV/
Vis irradiation, with the isomerization degree comparable to
visible light irradiation.
UV/Vis irradiation, likely due to the lower cross-linking degree
of siloxane networks. However, these changes were smaller than
those observed for F2 and F3, suggesting that solid-state reac-
tions gave siloxane networks which limited the mobility of
azobenzene moieties.
16
that of pendant-type hybrids.
XRD patterns of F2 and F3 before and aer UV/Vis irradia-
tions are shown in Fig. 9. For F2, UV irradiation caused a
disappearance of the XRD peaks and a further Vis irradiation
did not recover the lamellar structure (Fig. 9(a)). This order-to-
disorder transition upon UV irradiation was considered to be
caused by the partial trans-to-cis transition of azobenzene
4
Conclusions
We have demonstrated the formation of lamellar hybrid materials
from azobenzene-bridged ethoxysilane precursors. Three types of
photo-responsive precursors differing in the number of ethoxy
groups were used. The triethoxysilylated precursor, which gener-
ates the largest number of SiOH groups, had a strong tendency to
form self-assembled, lamellar structures. By employing an evap-
oration induced self-assembly process or a solid-state reaction
process, ordered lamellar structures were also prepared from di-
and mono-alkoxysilyl precursors. The photo-responsive proper-
ties of the hybrid lms were inuenced by cross-linking degree of
siloxane networks: trans–cis isomerization of azobenzene moie-
ties was improved by decreasing of Si–O–Si bonds via utilizing
precursors with less condensable ethoxy groups. Further struc-
tural design of such self-assembled hybrid materials will lead to
the creation of novel photo-responsive materials.
(described as above). Further polycondensation may occur aer
UV irradiation, xing the disordered structure, and thereby
inhibiting a reversible change into the initial ordered structure
even aer Vis irradiation (Scheme S1 in ESI†). Further poly-
condensation aer UV irradiation was conrmed by FT-IR
spectra (Fig. S7 in ESI†). On the other hand, neither a change in
the diffraction peak intensity nor position was observed for F3
upon UV/Vis irradiation (Fig. 9(b)). One possible explanation is
that F3 contains some disordered domains, where azobenzene
groups can more easily undergo photoisomerization.
0
0
0
UV-Vis spectra of F1 , F2 and F3 (Fig. S8 in ESI†) show large
absorption peaks for the trans isomers of azobenzene even aer
UV irradiation. There were no changes upon UV/Vis irradiation
0
16
for F1 , which is also in clear contrast to pendant-type hybrids.
0
The absorption peak of F1 at 330 nm was obviously blue shied
in comparison to the ethanol solution of 1 (Fig. 2), as well as to
the F2 and F3 lms. This suggested an enhanced H-aggrega-
0
0
Acknowledgements
32,33
0
tion of azobenzene moieties.
azobenzene groups and the more rigid siloxane networks
In F1 , the closer packing of
The authors thank Mr Masashi Yoshikawa (Waseda University)
for his help in the ESI-MS analysis. This work was supported in
part by a Grant-in-Aid for Scientic Research (C) from Japan
Society for the Promotion of Science (JSPS) and by a Grant-in-
Aid for Scientic Research on Innovative Areas ‘New Polymeric
Materials Based on Element-Blocks (no. 2401)’ provided by The
Ministry of Education, Culture, Sports, Science and Technology,
Japan. S.G. acknowledges the nancial support offered by the
Monbukagakusho Scholarship and by the Global Centre for
Excellence for Mechanical Systems Innovation (GMSI, The
University of Tokyo).
0
0
should hinder photo-isomerization. In contrast, F2 and F3
lms showed slightly reversible trans–cis isomerizations aer

Notes and references
Fig. 8 UV-Vis spectra of (a) F2 and (b) F3 films before irradiation (red
line), after 5 min of UV irradiation (blue line), and after a further 5 min of
visible light irradiation (green line).
1
F. Hoffmann, M. Cornelius, J. Morell and M. Froba, Angew.
Chem., Int. Ed., 2006, 45, 3216.
25324 | RSC Adv., 2014, 4, 25319–25325
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
2
3
4
(a) A. Mehdi, C. Reye and R. Corriu, Chem. Soc. Rev., 2011, 40, 17 M.-H. Li, P. Keller, B. Li, X. Wang and M. Brunet, Adv. Mater.,
5
63; (b) A. Chemtob, L. Ni, C. Croutx ´e -Barghorn and
2003, 15, 569.
B. Boury, Chem.–Eur. J., 2014, 20, 1790.
W. Chaikittisilp, M. Kubo, T. Moteki, A. Sugawara-Narutaki,
18 T. Ikeda, M. Nakano, Y. Yu, O. Tsutsumi and A. Kanazawa,
Adv. Mater., 2003, 15, 201.
A. Shimojima and T. Okubo, J. Am. Chem. Soc., 2011, 133, 19 T. J. White, N. V. Tabiryan, S. V. Serak, U. A. Hrozhyk,
13832.
V. P. Tondiglia, H. Koerner, R. A. Vaia and T. J. Bunning,
K. Kuroda, A. Shimojima, K. Kawahara, R. Wakabayashi,
So Matter, 2008, 4, 1796.
Y. Tamura, Y. Asakura and M. Kitahara, Chem. Mater., 20 M. Yamada, M. Kondo, J. Mamiya, Y. Yu, M. Kinoshita,
2
014, 26, 211.
C. J. Barrett and T. Ikeda, Angew. Chem., Int. Ed., 2008, 47,
4986.
21 M. Yamada, M. Kondo, R. Miyasato, Y. Naka, J. Mamiya,
M. Kinoshita, A. Shishido, Y. Yu, C. J. Barrett and T. Ikeda,
J. Mater. Chem., 2009, 19, 60.
5
6
A. Shimojima, Y. Sugahara and K. Kuroda, Bull. Chem. Soc.
Jpn., 1997, 70, 2847.
L. D. Carlos, V. D. Bermudez, V. S. Amaral, S. C. Nunes,
N. J. O. Silva, R. A. S. Ferreira, J. Rocha, C. V. Santilli and
D. Ostrovskii, Adv. Mater., 2007, 19, 341.
22 C. Kordel, C. Popeney and R. Haag, Chem. Commun., 2011,
47, 6584.
7
8
9
H. Peng, J. Tang, J. Pang, D. Chen, L. Yang, H. S. Ashbaugh,
C. J. Brinker, Z. Yang and Y. Lu, J. Am. Chem. Soc., 2005, 127, 23 S. S. Nobre, X. Cattoen, R. A. S. Ferreira, C. Carcel,
1
2782.
V. Z. Bermudez, M. Wong Chi Man and L. D. Carlos, Chem.
Mater., 2010, 22, 3599.
J. J. Moreau, L. Vellutini, M. Wong Chi Man, C. Bied,
J.-L. Bantignies, P. Dieudonne and J.-L. Sauvajol, J. Am. 24 B. Menaa, M. Takahashi, P. Innocenzi and T. Yoko, Chem.
Chem. Soc., 2001, 123, 7957. Mater., 2007, 19, 1946.
J. J. E. Moreau, B. P. Pichon, M. Wong Chi Man, C. Bied, 25 X. Zhou, S. Yang, C. Yu, Z. Li, X. Yan, Y. Cao and D. Zhao,
H. Pritzkow, J.-L. Bantignies, P. Dieudonne and
Chem.–Eur. J., 2006, 12, 8484.
J.-L. Sauvajol, Angew. Chem., Int. Ed., 2004, 43, 203.
0 J. J. E. Moreau, B. P. Pichon, G. Arrachart, M. Wong Chi Man
and C. Bied, New J. Chem., 2005, 29, 653.
26 G. Cerveau, S. Chappellet, R. J. P. Corriu, B. Dabiens and
J. L. Bideau, Organometallics, 2002, 21, 1560.
27 C. J. Brinker, Y. Lu, A. Sellinger and H. Fan, Adv. Mater.,
1999, 11, 579.
1
1
1 J. J. E. Moreau, L. Vellutini, M. Wong Chi Man, C. Bied,
P. Dieudonne, J.-L. Bantignies and J.-L. Sauvajol, Chem.– 28 L. Nicole, C. Boissiere, D. Grosso, A. Quach and C. Sanchez, J.
Eur. J., 2005, 11, 1527. Mater. Chem., 2005, 15, 3598.
2 F. Ben, B. Boury and R. J. P. Corriu, Adv. Mater., 2002, 14, 29 A. Shimojima and K. Kuroda, Chem. Rec., 2006, 6, 53.
1
1
1
1
1
1
081.
30 H. Muramatsu, R. Corriu and B. Boury, J. Am. Chem. Soc.,
2003, 125, 854.
31 B. Boury, F. Ben and R. J. P. Corriu, Angew. Chem., Int. Ed.,
2001, 40, 2853.
32 Z. T. Nagy, B. Heinrich, D. Guillon, J. Tomczyk, J. Stumpe
and B. Donnio, J. Mater. Chem., 2012, 22, 18614.
33 C. L. Yeung, S. Charlesworth, P. Iqbal, J. Bowen,
J. A. Preeceb and P. M. Mendes, Phys. Chem. Chem. Phys.,
2013, 15, 11014.
3 N. Liu, K. Yu, B. Smarsly, D. R. Dunphy, Y.-B. Jiang and
C. J. Brinker, J. Am. Chem. Soc., 2002, 124, 14540.
4 M. Alvaro, M. Benitez, D. Das, H. Garcia and E. Peris, Chem.
Mater., 2005, 17, 4958.
5 E. Besson, A. Mehdi, D. A. Lerner, C. Reye and R. J. P. Corriu,
J. Mater. Chem., 2005, 15, 803.
6 S. Guo, A. Sugawara-Narutaki, T. Okubo and A. Shimojima, J.
Mater. Chem. C, 2013, 1, 6989.
This journal is © The Royal Society of Chemistry 2014
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